1. Field of the Invention
The present invention relates to operation of the hot gas generator for a dryer system, particularly the dryer system for a pulverizing mill assembly.
2. The Prior Art
It is known to incorporate a dryer arrangement in a comminuting or pulverization system, such as a coal briquetting mill assembly. Generally, pulverization systems including a dryer arrangement may be called "grinding dryer assemblies".
A grinding dryer assembly requires hot gases for drying. These gases are produced by a hot gas generator. In some cases, the hot gases are diverted from the exhaust flow of a combustion unit functioning in the system. Hot gas generators require a supply of primary combustion air, which is oxygen-rich, for generating a flame and a flow of secondary air for cooling the combustion gases and setting the desired temperature for the hot gas to be delivered.
Typically in the prior art, secondary air is supplied from the ambient as, for example, shown in German Auslegeschrift No. 1,283,655. The result is a hot gas supply having a relatively high oxygen content, for example on the order of 14 through 18 percent oxygen, which may cause afterburning. A further drawback in providing ambient secondary air is that relatively cool environmental air has a quenching effect as the combustion process, resulting in higher combustion operation costs.
In contrast to drawing secondary air from ambient, the hot gas generator can be placed in the return gas line for a grinding dryer assembly as, for example, shown in German PS No. 475815. However, the hot gas generator must have an extremely large flow cross section to accommodate throughput of the entire return gas flow of the assembly, which represents an extravagent need for space and high manufacture costs. A further disadvantage with this approach occurs in that the operations and controls for the hot gas generator and grinder dryer system become interrelated, since there is a functional coupling of the two devices, which makes individual control of each device difficult. In addition, solid particles entrained in the assembly gas flow may be combusted in the hot gas generator, especially coal dust particles, which interferes with regulation of the generator. During start up of the circulation system using this approach, there is an increased risk of explosion in the hot gas generator since gas flow through the generator does not occur until a short time after the circulation blower is activated, due to inertia.